In semiconductor lithography, device component features of an integrated circuit should be produced in a manner that is insensitive to process variation, such that the finished devices function within design tolerance. The lithographic process must also have sufficient throughput to meet market demand while maintaining a cost-per-device that is similar to that of all preceding generations. For semiconductor manufacture to remain profitable, the amount of functionality on a chip of a given size ideally doubles every 18 to 24 months. This requirement is equivalent to a doubling of the number of component features that can be fabricated on a chip in every cycle of Moore's Law, which provides that the density of transistors in integrated circuits doubles every two years (Moore, G. E. (1965) “Cramming more components onto integrated circuits,” Electronics, 38(8):114-117).
As known in the art, the resolution in conventional photolithographic techniques is directly proportional to the wavelength of light employed for exposure. A commonly used guideline for the diffraction limit is the Abbe criterion:
      d    =          λ              2        ⁢        NA              ,where d is the transverse resolution (defined as the shortest distance between resolvable features in imaging and the shortest repeat distance for printable features in photolithography), λ is the wavelength of light, and NA is the numerical aperture of the imaging system. The NA is defined by the product of the refractive index of the medium and the sine of the half-angle over which light is collected (in the case of imaging) or focused (in the case of photolithography).
This relationship has created great challenges for the continued progress of Moore's Law, as conventional photolithographic approaches are not focusing on the use of extreme ultraviolet (EUV) light, given EUV is expensive and difficult to generate, propagate and manipulate. As such, the predominant strategy for maintaining the course of Moore's law in semiconductor lithography has been to find ways to perform photolithography with ever shorter wavelengths of light. However, each reduction in wavelength brings its own set of challenges in optics and material chemistry. As the wavelength of light becomes shorter, generating, propagating, and manipulating the light becomes increasingly difficult and expensive. Moreover, the light tends to interact with virtually every material, requiring that propagation occur in a vacuum and that special, relatively expensive optics be used for manipulation. As such, it becomes more challenging to develop suitably robust resist materials.
Thus, particular attention is given to the physics, chemistry and engineering of imaging. Engineering approaches seek to provide the best quality image of the desired pattern in the medium, and therefore a detailed knowledge of how that image is recorded is required. The final developed photoresist image is a function of the contrast of the projected image as recorded through the thickness of the photoresist film, the density and uniformity of the photoevents needed to produce the image, and the development contrast of that recorded medium. For engineering process purposes, it is convenient to know the contrast of the projected image of the photomask and how that contrast translates into: (1) the image that forms in the resist film; (2) the latent image that is subsequently recorded; (3) the latent image formed after the post-exposure bake process; and (4) the develop-rate response to the integrated exposure and the thermal dose profile.
The image is formed by transmission of light through, or reflection of light from, a pattern of the objects to be imaged, or photomask pattern. The photomask pattern is focused onto a substrate using a projection lens which forms an image, referred to as the aerial image. The aerial image forms when portions of the diffraction pattern that arise from the interaction of the exposure radiation with the photomask pass through the projection lens and converge at the best focus at the image plane, interfering constructively. Most instances in semiconductor lithography do not involve point convergence. Instead, the image that is actually recorded in the photoresist is a convolution of the diffraction pattern and the pupil function (σ=sin(αiII)/NA), where αiII is the maximum half-cone angle of the illumination impinging on the mask objects, and σ represents the partial coherence of the system). The image can be degraded by lens aberrations and/or modified by pupil filters. Ideally, the aerial image forms a square-wave intensity profile, but at extreme resolution the image quality is lost because the NA filters out higher diffraction orders. The loss of the electric field information contained in these higher diffraction orders results in a degraded image.
With respect to chemistry, the photoresist material acts as a threshold detector of the aerial image as recorded in the film. With little to no exposure, no visible change is observed in the material after development. As the exposure dose increases, a threshold is reached in which there is a clear physical response in the material. This threshold is attained first at the intensity maxima of the image, where the shape of the object to be printed is first realized. As the exposure dose is increased further, more regions of the recorded image distribution in the resist reach this threshold. However, if there is continued exposure, even the regions that receive minimum intensity surpass the threshold. Such overexposure results in the loss of the image to be printed and/or feature resolution.
Typically, the lithography engineer samples a narrow range of aerial image intensity in the neighborhood of the final feature size, ultimately creating a developed and etched device component of the desired size and shape. In the region in which the latent image is sampled to form the final developed and etched component feature, the photoresist development response is usually nonlinear. The response is not binary, however, so there is a response in the subthreshold regions that depends on the resist's development rate with respect to the integrated dose in that region of the latent image. Because of this phenomenon, the latent image is typically not sampled at the image width that matches the desired size to yield that target. Instead, one uses a somewhat lower dose for positive-tone resists and a somewhat higher dose for negative-tone resists. The exposed resist is then etched back to the target size using a developer. Etch-back depends on the image recorded in the resist, with low-contrast images having larger bias than high-contrast images for the same developer processing conditions.
The complex relationship between physical and chemical contrast is at the heart of the ultimate production resolution for a given set of imaging conditions. It is not enough simply to produce an image; the image must also have an adequate contrast to interact with the resist contrast. Wavefront and resist engineers therefore pay particular attention to the dependence of the resist response on the location where the latent image is sampled, on the exposure dose, and on the sampling of other sets of latent images generated with changes in focus, changes in pitch, and changes in duty cycle (the line:space ratio). The goal is to maximize process tolerance for all features being imaged across some common process window of focus and dose variation that has been determined to be required to produce the target yield in the production of integrated circuits.
The usable contrast that can be sampled to form the image depends on how the resist responds to changes in contrast for the portion of the aerial image that is sampled to produce the resultant image. The response to focus variation in terms of feature size, line edge roughness and image placement must be minimized for all features of interest. The resultant half-pitch of the transverse resolution may be described using a variant of the Rayleigh equation (CD=k1(λ/NA), where k1 is a process parameter). Resolution to a semiconductor lithographer is the smallest pitch that is imaged with a manufacturable process for a feature of that pitch, rather than the smallest single printable image.
The ongoing need to create smaller features has driven the approaches to imaging with smaller λ and larger NA in order to enhance resolution. These approaches have been supplemented by decreasing k1 with the use of: (1) partially-coherent illumination tuning; (2) strong phase-shift masks and low-partial-coherence, on-axis illumination; or (3) weak phase shift masks, in particular with on-axis and off-axis illumination. Such approaches may also be supplemented or enabled by the use of optical proximity correction and assist features to tune the diffraction pattern, such that it provides the optimum interference to produce images with the highest fidelity under the specific imaging conditions.
An alternative class of approaches to improving photolithographic resolution has been developed which rely on the use of two colors of light, one of which exposes a photoresist and another of which acts to counteract or inhibit this exposure (see Fourkas, J. T. and Petersen, J. S. (2014) “2-Colour photolithography,” Phys. Chem. Chem. Phys. 16:8731-8750); Fourkas, J. T. (2011) “RAPID Lithography: New Photoresists Achieve Nanoscale Resolution,” Opt. Photon. News 22, p. 24). Such approaches provide for the use of visible light (e.g., electromagnetic radiation having wavelengths in the range of 400 nanometers (nm) to 700 nm), which is inexpensive to generate, propagate and manipulate, to create features with nanoscale resolution. Thus, the key feature of a two-color photoresist is that it responds in a different manner to two different light sources, which are typically at two different wavelengths. Two-color photolithography approaches provide for negative-tone photoresists, in which case an excitation light source is used to initiate crosslinking in the photoresist and a deactivation light source is used to prevent or to quench the crosslinking. The spatial distributions of the two light sources differ in a manner designed to minimize the dimensions of the features that are fabricated.
Two-color approaches to photolithography have relied on a range of different mechanisms for deactivation, including stimulated emission, absorption modulation, photoinduced radical quenching, photoinduced back-transfer of electrons, and reverse intersystem crossing (RISC). Typically, one color of light is first used to excite the photoinitiator (PI) molecules in the photoresist in a desired pattern. A second color of light, which is of a longer wavelength than the first color of light, is used to drive the PI molecules back to the ground state before they have had the chance to cause chemistry to occur within the photoresist.
In such two-color approaches, the range of materials that undergo these processes without undesirable side reactions has been very limited. Furthermore, the pattern of the deactivation is typically complementary to the pattern of excitation, such that the photoresist ends up being exposed effectively only in the regions in which the intensity of the deactivation beam is at a minimum. The materials reported to date have suffered from the shortcoming that the state that is deactivated also leads to reaction, which means that these two processes compete with one another. This competition invariably leads to the buildup of background exposure when tightly-packed features are created, ultimately limiting the resolution that can be attained using such approaches.
Thus, a limitation of such conventional two-color approaches is that, although features can be much smaller than either of the wavelengths of light employed, the minimum pitch (i.e., the distance between two features) is determined by the wavelength used for deactivation. As such, multiple patterning steps are required in two-color approaches in order to obtain densely packed features. However, in such two-color approaches, deactivation and chemistry both occur from the same state, and so these processes necessarily compete with one another. As a result, it is not possible to completely deactivate a region of the photoresist that has been excited. This phenomenon leads to a degradation of the resolution as the number of patterning steps increases.
Thus, there is a need for improved photoresist compositions and methodologies for high-resolution photolithography which overcome some or all of the above-noted problems.